A lot of people find Enthuware's mocks to be among the highest quality and the least expensive, simultaneously.
While it would logically make sense to spend more than those cost even, considering the economic cost of failing an exam due to being unsure if one is prepared or not...
I am not sure which other ones I like, if you search these very forums you will find feedback from a good number of people reviewing them as consumers.
Stuff like "Finally passed after failing the first time. This time I used X and Y to prepare, instead of just Z like the first time which gave me a false sense of security going in as it was too easy."
Before I take the 819 I fully expect to get the Sybex book of mock exams for that as well. It costs more than the Enthuware but so much less than the money wasted on a failed exam...
Anyway, read what people have posted here, especially people who thought about it after passing and strongly felt source A, B or C was extremely helpful.
I will mention that there is a lot of pretty low-quality stuff around too, especially for free. I wasted a bunch of time before I got serious and started coming here, I mean, I'd find several errors in things on topics I didn't even consider myself to know yet...maybe I should have asked for my $0.00 back, right?